The present research will examine hormonal organization of sexually dimorphic play in juvenile rats. Previous work indicates that males exhibit more rough play than females and that neonatal testosterone treatment elevates rough play in both sexes. Three questions will be asked in the present investigation: 1) Are endogenous testicular hormones responsible for sex differences in rough play? 2) When during the perinatal period does hormonal organization occur? and 3) By what mechanism does testosterone exert its apparent organizational effects? Specifically is the aromatization of testosterone to estradiol, which is responsible for defeminization, important for this organizational effect? Alternatively, testosterone might act directly or via conversi n to another androgen. This mode of organization seems important in the organization of male sexual behavior and avoidance behavior in the rat and sexually dimorphic play in rhesus monkeys. A combination of exogenous testosterone and antiandrogen treatment and gonadectomy at different ages will be used to address the first two questions. Subsequent studies using injections of estradiol, dihydrotestosterone and agents which inhibit aromatization will focus on the biochemical mechanisms underlying organization of sexually dimorphic play.